1. Field of the Invention
The present invention relates to a state detecting apparatus for detecting the state of an electrical device or switch.
2. Description of the Prior Art
In general, a potential detecting circuit is used for detecting the state of a switch or other electrical device. FIG. 1 shows a circuit for detecting the state of a switch. The apparatus of FIG. 1 will be briefly described. In FIG. 1, the reference numeral 1 designates a switch; 2 designates a wire; 3 designates a state detecting circuit; 4 designates a terminal for an electric power source (not shown); 5 designates an input terminal; 6 designates an output terminal; and 8 designates a potential detecting circuit. The wire 2 connects the switch 1 to the state detecting circuit 3. The wire 2 is connected through the resistance 7 to the terminal of electrical power source 4 whereby the potential of the input terminal 5 is kept at the potential of the power source, i.e. "H", in the OFF state of the switch 1. When the switch 1 is in the ON state, the input terminal 5 is grounded and is in the "L" state.
The potential detecting circuit 8 detects the OFF state or ON state of the switch by detecting the change of the switch 1 from the OFF state to the ON state, i.e. by detecting the change of potential of the input terminal 5 from the "H" to the "L" state. The potential detecting circuit 8 detects whether the potential applied to the input terminal 5 is the potential of the power source or the ground potential. The potential detecting level can be in the range of the ground potential to the potential of the power source.
In general, as shown in FIG. 1, the wire 2 for grounding the switch 1 and the state detecting circuit 3 is rather long which can result in a disconnecting fault. When the wire 2 is disconnected and the information concerning the switch 1 is not accurately transmitted to the state detecting circuit 3, the reliability of the control system is quite low even though the reliability of the control device operated by the switch may be quite high.
This problem is particularly acute in a vehicle where many switches are disposed in various positions of the vehicle and some switches are connected to the engines. In this case, the outer environment around the engine is not stable with the result that a wire fault may be easily caused and a detection of the fault is rather difficult.